The attack occured at 2 p.m. (0500 GMT), the state-run Korea Information Security Agency said, and some hours later some systems were still down. Two South Korean banks, Shinhan Bank and Nonghyup, and three TV stations KBS, MBS and YTN, all reported that their networks had suddenly shut down on Wednesday afternoon.

The computer shutdown did not affect government agencies and there were no reports that bank customers' records were compromised.

SKorean officials believe that a "malicious" code was to blame for the system failure. Investigators are trying to identify and analyse the virus.

Last week, North Korea accused the US and its allies of attacks on its internet servers.